1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic printing method and the like.
2. Description of the Related Art
A printer will be described as an example among image forming apparatuses.
As a method of charging an image bearing member of an image forming apparatus with a charging member, such as a charging roller, a DC charging bias is applied to the charging roller by a constant-current-control. By making constant-current-control, good charging can be made without being affected by fluctuations in the impedance of the charging roller or environmental fluctuations.
FIG. 12 illustrates a schematic arrangement of a conventional charging bias application circuit 1301. A voltage setting circuit portion 1302 changes a set value in response to a PWM signal. The charging bias application circuit 1301 includes a transformer driving circuit portion 1303 and a high voltage transformer 1304. A feedback circuit portion 1305 converts the value of an electric current I162 passing through a charging roller (a charging member) 106 to a voltage with a resistor R161 for detection, and transmits this voltage to an engine controlling portion as an analog value from J501. Then, based on this analog value, the engine controlling portion sets a PWM signal so as to be a required electric current value. Making a series of control in such an arrangement can pass a constant electric current value through the charging roller. Application of such an embodiment is made in Japanese Patent No. 3397339. Like this, by applying a DC charging bias by constant-current-control, images of a constant density, without being affected by fluctuations in impedance of a charging roller or environmental fluctuations, can be obtained.
When applying a DC charging bias by constant-current-control, if remaining electric potential is left on an image bearing member, an electric potential difference is decreased between a charging member and the image bearing member, and an electric current is less likely to pass. By making constant-current-control in such situations, the electric potential of the charging member 106 will be set to be excessively high, and thus the electric potential of the image bearing member to be charged will be higher as well, eventually causing poor imaging. Therefore, as to the remaining electric potential, an electric charge needs to be eliminated using an optical element (charge eliminating device) such as an LED to reduce an electric potential. Thus, this charge eliminating process needs to be inserted in sequence. However, in the case of the occurrence of deterioration, contamination or breakdown in the charge eliminating device, applying a DC charging bias by constant-current-control as it is may cause poor imaging.
This deterioration or the like of the charge eliminating device may not be found even if an electric current passing through the charge eliminating device is detected. For example, when a charge eliminating device is an optical element, the surface of the optical element may be contaminated with toner. In this case, in spite of the same electric current as in the normal state passing through the optical element, a sufficient exposure cannot be made onto an image bearing member, resulting in the occurrence of charging failure, and thus poor imaging.